The invention relates to the field of generating broadband white light. The devices used generally comprise a microstructured optical fiber of structure that is adapted to generate white light, the fiber being fed by a coherent light source emitting at a given wavelength.
Microstructured optical fibers are also known as photonic crystal fibers (PCF), holey fibers or hole-assisted fibers.
In general, the coherent light source used is a laser, emitting light pulses at a certain repetition frequency.
Microstructured optical fibers are generally constituted by cladding in which there is a fiber core surrounded by microstructures.
In a cross-section of the fiber, these microstructures are formed by holes, each extending along the fiber and arranged in a particular layout, which layout is characterized in particular by the diameter of the holes and the pitch between the centers of two neighboring holes.
The fiber core may be made of a material that is different from that of the cladding, however it is usually made from the material of the cladding, with the shape of the fiber core being determined merely by the presence of the holes that surround it.
Microstructured optical fibers provide numerous advantages compared with conventional optical fibers, which are constituted by a fiber core made of doped silica, said core being surrounded by cladding having a refractive index that is slightly lower than that of the fiber core.
The large difference in refractive index between the core (made of silica or doped silica) and the cladding (made of silica with air holes) makes it possible to design microstructured optical fibers with a core diameter that is very small.
Furthermore, microstructures provide numerous design options.
Within a section of a microstructured optical fiber, it is possible to modify hole diameter, pitch between neighboring holes, the type of mesh formed relative to one another by the holes (triangular, hexagonal, etc. . . . , mesh), and also the shape and the outside dimension of the structure formed by the set of holes.
In addition, it is also possible to cause certain geometrical characteristics to vary along the microstructured optical fiber in order to obtain certain properties. For example, it is possible to cause the diameter of the core to vary along the fiber by modifying the outside diameter along the fiber. This adaptation is commonly referred to as “tapering”, and sometimes as “conicity”.
Advantage has thus been taken of the dimensioning options made available by microstructured optical fibers for the purpose of generating broadband white light, also referred to as a “supercontinuum”.
For example, the small diameters of microstructured optical fiber cores compared with the diameters of conventional optical fiber cores enhance non-linear phenomena. Such non-linear phenomena are sought after when it is desired to generate white light.
By way of example, the applications that can be envisaged with a broadband white light source include confocal fluorescent microscopy, flux cytometry, spectroscopy, tomography, inspecting semiconductors, or characterizing optical fibers.
In order to enhance the generation of broadband white light, various techniques have already been proposed.
One technique used for making it easier to obtain broadband white light is to enhance highly non-linear behavior by doping the core of a microstructured optical fiber.
In this respect, germanium doping is particularly advantageous.
With germanium doping, non-linear processes within the optical fiber are increased, by increasing both the non-linear refractive index coefficient (n2) of the fiber core and the Raman gain coefficient (CR).
The non-linear refractive index coefficient n2 is involved in defining the non-linear coefficient g of the fiber, using the following relationship:γ=2πn2/λSeff  (R1)                where l is the wavelength under consideration and Seff is the effective area of the microstructured optical fiber for the propagation mode under consideration.        
The influence of germanium doping in an optical fiber core on the non-linear refractive index coefficient is disclosed for example in the document “Dopant dependence of effective non-linear refractive index in Ge and F doped core SMF”, by K. Nakajima, Photonics Technology Letters, Vol. 14, Issue 4, pp. 492-494, April 2002.
Increasing the non-linear refractive index coefficient contributes to increasing non-linear effects such as cross-phase modulation, self-phase modulation, or four-wave mixing.
A high Raman gain coefficient represents a high rate of energy transfer from the incident pump wave to the light conveying the optical signal. Simulated Raman diffusion enables the light serving to convey the optical signal to be diffused from the incident energy of the pump wave.
It should be observed that stimulated Raman diffusion can be observed regardless of the pump wavelength.
Stimulated Raman diffusion gives rise to a wavelength shift between the pump wave and the light conveying the optical signal. This shift may take place towards longer wavelengths (red shift also referred to as Stokes shift).
The use of a microstructured optical fiber having its core doped with germanium to obtain a Raman amplifier is described for example in U.S. Pat. No. 7,136,559.
Nevertheless, U.S. Pat. No. 7,136,559 does not specifically describe a broadband white light generator.
However it has been suggested that microstructured optical fibers with a core that can be doped with germanium might be used for obtaining light presenting a spectrum that extends over an enlarged band of wavelengths.
This applies for example to document US 2004/0105640. US 2004/0105640 describes a microstructured optical fiber comprising first cladding having holes of a first diameter surrounding the fiber core and second cladding provided with holes of a second diameter, which holes are arranged in the form of a regular mesh around the first cladding.
That structure enables low chromatic dispersion to be obtained over an enlarged band of wavelengths, with an effective area that is small, and without implementing holes that are too small.
The results described show that microstructured optical fibers make it possible, at their outlets, to obtain light of spectrum that covers the band 1200 nanometers (nm) to 1700 nm using a light source that emits at 1550 nm and in the absence of any doping. Nevertheless, no result is described for a microstructured optical fiber that is doped.
The same applies to document US 2005/0238307. That document proposes a microstructured optical fiber that likewise enables light to be obtained that is spread over an enlarged band of wavelengths, typically over the range 1200 nm to 1700 nm.
The light source emits at a wavelength of 1550 nm, that matches the zero dispersion wavelength lZDW of the fiber. The zero dispersion wavelength (no chromatic dispersion) is the wavelength at which the dispersion effects associated with the material forming the core of the optical fiber cancel the effects of guidance dispersion, which wavelength depends in particular on the diameter of the fiber core.
To this end, US 2005/0238307 proposes a structure comprising first cladding with elements surrounding the fiber core and second cladding having holes that are arranged in the form of a regular mesh around the first cladding. The elements of the first cladding present a refractive index lower than the refractive index of the core but higher than the refractive index of the holes in the second cladding.
With that structure, the refractive index around the core is increased in comparison with a structure that does not include first cladding.
Consequently, the slope of the variation in chromatic dispersion around the zero dispersion wavelength is reduced, thereby making it possible to obtain a fiber with dispersion that is zero or close to zero over a wider band of wavelengths.
Unlike document US 2004/0105640, document US 2005/0238307 presents a variant of the microstructured optical fiber structure that has a fiber core that is doped with germanium.
In that variant, the elements of the first cladding and the holes in the second cladding are similar, containing air or a vacuum, and they are arranged in such a manner that d/A lies in the range 0.44<d/A<0.56, where d is the hole diameter and A is the spacing between two neighboring holes, A lying in the range 1.24 micrometers (μm) to 1.61 μm. This produces a structure that is simpler, in which the first cladding presenting elements with a suitably selected refractive index is omitted in favor of doping the core with germanium, thereby also having the effect of increasing the refractive index of the fiber core.
In addition, US 2005/0238307 specifies that the microstructured optical fiber doped in that way with germanium presents behavior in which the fundamental mode is confined within the fiber core at the wavelength l=1550 nm. Insofar as light propagation takes place in the fundamental mode, the broadband white light coming from the optical fiber is of good quality.
That result complies well with the available theoretical data when the fiber presents linear behavior, when the pump wavelength of 1550 nm is relatively high, and when the ratio d/A is relatively low, thus enhancing propagation on the fundamental mode.
The germanium-doped microstructured optical fiber proposed in US 2005/0238307 thus makes it possible to obtain good quality light at the outlet from the fiber (propagating on the fundamental mode), but over a bandwidth that remains limited to the range 1200 nm to 1700 nm.
Nevertheless, it has been possible to obtain a source of white light presenting a broader bandwidth, e.g. over the range 500 nm to 1700 nm, when using a germanium-doped microstructured optical fiber.
By way of example, one such microstructured optical fiber is disclosed in “Microstructured fibers with highly non-linear materials” by Schuster et al., Opt. Quant. Electron (2007), 39: pp. 1057-1069.
For this purpose, Schuster et al. propose doping the fiber core at a very high concentration, with the maximum refractive index difference between the doped fiber core and the silica of the cladding being greater than 55′10−3. This enhances non-linear effects in the fiber, and consequently assists in obtaining a spectrum of enlarged width at the outlet from the optical fiber.
Furthermore, the microstructured optical fibers that were tested presented a ratio d/L (where d is the diameter of the hole and L is the pitch between respective centers of two neighboring holes) that was likewise high, such that 0.87<d/L<0.9.
Given these characteristics (high doping level, high d/L ratio), the diameter of the fiber core is too great to enable propagation on a single mode. The white light obtained at the outlet from the optical fiber thus propagates on a plurality of propagation modes.
Schuster et al. appear to present such propagation on a plurality of modes as a consequence of the very high level with which the optical fiber core is doped with germanium that is of interest for the purpose of generating a broadband white light spectrum, with this taking advantage of the blue shift of the zero dispersion wavelength with increasing propagation mode order.
Nevertheless, it can be observed that the fact that the white light is generated on a plurality of propagation modes degrades the quality of the signal. The flatness of the power of the signal at the outlet from the optical fiber (FIG. 6 in Schuster et al.) is poor. For example, for the 80 μm fiber, flatness over the spectrum as a whole is about 10 decibels (dB) in the range 600 nm to 1700 nm. The level of flatness is greater than 10 dB for the fiber having a diameter of 125 μm, with two distinct power levels on either side of the pump wavelength. The signal at the output from the fiber is thus difficult to use as a white light source, and this applies to most of the applications that might be envisaged for using a broadband white light source.
By way of example, another technique that has been proposed is described in the document “Zero-dispersion wavelength decreasing photonic crystal fibers for ultraviolet-extended supercontinuum generation”, by Kudlinski et al., Optics Express, Vol. 14, Issue 12, pp. 5715-5722, May 2006, which proposes a non-doped microstructured optical fiber and in which the diameter of the optical fiber core decreases along the fiber from an inlet value while conserving a constant void fraction (ratio of the overall section of the microstructures divided by the overall section of the optical fiber) all along said optical fiber.
The value of the diameter of the core of the microstructured optical fiber at its inlet is selected to initiate generation of the broadband continuous spectrum in a configuration where the zero dispersion wavelength is matched to the pump wavelength (i.e. about 1 μm in that example), which implies a certain diameter for the core of the optical fiber.
Thereafter, the decrease in the diameter of the core of the microstructured optical fiber (“taper”) combined with conserving a constant void fraction all along the optical fiber makes it possible to decrease the zero dispersion wavelength along the fiber and to enhance the generation of wavelengths shorter than the pump wavelength so as to extend the bandwidth towards the blue and ultraviolet regions of the spectrum.
That other technique thus seeks to modify the effective area Aeff of the microstructured optical fiber along the length of the fiber for the purpose of modifying the non-linear coefficient g of the fiber. The non-linear refractive index coefficient n2 corresponds to that of non-doped silica.
That makes it possible to obtain a white light source with a bandwidth covering the range 350 nm to 1750 nm.
Furthermore, light propagates in the microstructured optical fiber on the fundamental mode over the entire width of the band. The white light obtained at the outlet from the fiber is thus of good quality.